Thixotropic gel for vadose zone remediation

ABSTRACT

A thixotropic gel suitable for use in subsurface bioremediation is provided along with a process of using the gel. The thixotropic gel provides a non-migrating injectable substrate that can provide below ground barrier properties. In addition, the gel components provide for a favorable environment in which certain contaminants are preferentially sequestered in the gel and subsequently remediated by either indigenous or introduced microorganisms.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.12/074,735, filed on Mar. 6, 2008, which is now U.S. Pat. No. 7,896,577,which claims the benefit of U.S. Application Ser. No. 60/905,158, filedon Mar. 6, 2007, and which is incorporated herein by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under Contract No.DE-AC0996-SR18500 awarded by the United States Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

This invention is directed towards materials which may be introduced inor near a vadose zone to improve remediation efforts and efficiency withrespect to vadose zone contamination.

BACKGROUND OF THE INVENTION

The use of silica solutions and silica gels to solidify soil and rendersoil impermeable are well known in the art. U.S. Pat. No. 3,552,130entitled, “Method of Forming a Substantially Liquid Impervious Wall inan Earth Formation”, and which is incorporated herein by reference,discloses introducing materials into a soil in which a filler materialis mixed with a silica gel to form a material which will render certainsoil types impermeable.

U.S. Pat. No. 3,375,872, entitled “Method of Plugging or SealingFormations With Acidic Silicic Acid Solutions”, and which isincorporated herein by reference, discloses a method of sealing earthformations using a low pH silicic acid solution.

It is also known in the art to introduce oils such as a vegetable ormineral oil into a below ground contaminated area. The oil serves toconcentrate and remove organic contaminants from the soil so that thecontaminants may be removed by recovery of the oil or may be metabolizedby microorganisms which may be introduced into the oil fraction. Onesuch use of this technology is seen in U.S. Pat. No. 5,265,674 entitled“Enhancement of in Situ Microbial Remediation of Aquifers”, and which isincorporated herein by reference.

One difficulty of contamination remediation in the vadose zone is thetendency for treatment materials to flow straight down with littlelateral spreading. In addition, low viscosity materials leave lowconcentrations of the treatment materials in the soil pore spaces.Higher viscosity materials increase the saturation of the materialswithin the soil pore spaces but are difficult to emplace within asignificant sized volume needed for typical remediation environments.

Other difficulties of contamination remediation in the vadose zone isthat the vadose zone tends to be highly gas permeable, has a moisturecontent that is less than saturation, and a pressure that varies due toatmospheric pressure variations which create significant gas fluxes. Thevadose zone can be highly permeable to the influx of rain and surfacewater which can leach contaminants into the groundwater. Oxygen andother gases enter the vadose zone through the percolation of rain andsurface water and through gas exchange by diffusion and from barometricpressure differentials with the surface. As a result of theseproperties, it is difficult to perform certain bioremediation andphysical techniques within the vadose zone given the permeability, andthe open, porous nature of the vadose region.

Accordingly, there remains room for improvement and variation within theart.

SUMMARY OF THE INVENTION

It is one aspect of at least one of the present embodiments to provide athixotropic mixture of water and oil which will form a gel within anotherwise permeable vadose zone. The non-toxic gel of water and oil maybe created having a wide range of viscosities tailored to the specificpermeability of the vadose zone so as to provide a specific gelsaturation value within the soil and a barrier within the vadose zone.The permanence of the barrier may be controlled by application ofsurfactants.

It is another aspect of at least one of the embodiments of the presentinvention to apply within a vadose zone a non-migrating stable gel of athixotropic mixture of water and oil. The resulting gel, having anon-polar oil constituent, will preferentially attract and sequester anumber of non-polar contaminants within the vadose zone. Once thecontaminants are within the thixotropic gel, the contaminants are lesslikely to migrate and contaminate groundwater.

It is another aspect of at least one of the embodiments of the presentinvention to introduce within a vadose zone a non-migrating stable gelof a thixotropic mixture of water and oil. Once the contaminants arewithin the thixotropic gel, the contaminants are more amenable tobioremediation by native in situ microorganisms or through theapplication of other microorganisms by providing a carbon source (oil)for the microbes to grow and water for the microbes to live in.

It is another aspect of at least one of the present embodiments of theinvention to provide a thixotropic, non-toxic gel of water and oil thatmay be introduced into a vadose zone. The resulting gel creates ananaerobic environment within the vadose zone, the anaerobic environmentbeing beneficial for supporting desirable microorganisms that canbioremediate contaminants present within the vadose zone.

It is another aspect of at least one of the present embodiments of theinvention to provide a thixotropic, non-toxic gel of water and oil thatmay be introduced into a vadose zone. The resulting gel creates apersistent and durable barrier to trap downward migrating contaminantsand decrease water infiltration.

It is yet another aspect of at least one embodiment of the presentinvention to provide for a process of using a fumed silica incombination with an oil/water mixture to create a thixotropic materialthat may be injected as a liquid into a vadose zone and thereafter formsa substantially more viscous gel. The resulting gel has a long termresidence time within the vadose zone allowing migration of certaincontaminants such as chlorinated solvents into the gel. Once presentwithin the gel, indigenous or supplemental microorganisms can moreeffectively degrade the contaminants. The size, density, and physicalproperties of the resulting gel may be adjusted to bring about anaerobicconditions in the vadose zone where such conditions are useful for thegrowth and maintenance of microorganisms useful for bioremediationefforts.

It is yet another aspect of at least one of the present embodiments ofthe invention to provide a thixotropic, non-toxic gel of water and oilcontaining certain amendments and which may be introduced into a vadosezone. The resulting gel and the amendments therein may be used tocontrol the geo chemistry of the vadose zone. For instance, microbialactivity and the resulting changes of pH and redox potential canfacilitate the remediation and/or sequestering of certain metals.

It is yet another aspect of at least one of the present embodiments ofthe invention to provide a thixotropic, non-toxic gel of water and oilhaving amendments therein can be introduced into a vadose zone. Theresulting gel contains macro and micro nutrients to increase themicrobiological activity in the vadose zone. Macronutrients such asnitrogen and phosphorus may be incorporated into the gel in specificratios to control the amount of biomass formed.

It is yet another aspect of at least one of the present embodiments ofthe invention to provide a thixotropic gel for injecting into asubsurface soil comprising: about 40 percent to about 60 percent byvolume of a biodegradable vegetable oil such as soybean oil or mineraloil and combinations thereof; about 40 percent to about 60 percent byvolume of water which may be comprised of additional water solublecarbon sources such as sugars, lactate, etc.; and, fumed silica in anamount of between about 0.75 percent to about 2.0 percent by weight. Thethixotropic gel may further comprise nutritional organic supplements tosupply nitrogen, phosphorus, and other nutrients for supportingmicrobial growth such as yeast extracts, vitamins, corn steep liquor,seaweed extracts, and organic and inorganic fertilizer species andcombinations thereof.

It is yet another aspect of at least one of the present embodiments ofthe invention to provide a process of bioremediation within a vadosezone comprising: identifying a vadose zone contaminant area; creating athixotropic injectable solution comprising a mixture of an oil, water,and fumed silica; agitating the thixotropic injectable solution, therebyenabling the thixotropic injectable solution to be injected underpressure to a subsurface vadose zone; injecting the thixotropic solutioninto the vadose zone, said injectable solution thereby forming anon-migrating gel within the vadose zone; wherein the non-migrating gelestablishes an in situ anaerobic environment in response to biologicalcolonization within the non-migrating gel. The process of creating athixotropic injectable solution may include sparging the solution withnitrogen gas to remove free oxygen.

It is yet another aspect of at least one of the present embodiments ofthe invention to provide a process of remediating volatile organiccompounds within a vadose zone comprising: injecting a thixotropicmaterial into a vadose zone contaminated with volatile organiccompounds, the thixotropic material comprising a vegetable oil such assoybean oil or mineral or animal oil and combinations thereof;sequestering within the oil the non-polar volatile organic compoundspresent within the vadose zone; establishing anaerobic conditions withinthe thixotropic material; and, degrading said volatile organic compoundsby the metabolic activity of bacteria present within the thixotropicmaterial.

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A fully enabling disclosure of the present invention, including the bestmode thereof to one of ordinary skill in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying drawings.

FIGS. 1 through 4 are graphs depicting characteristics of thixotropicgels when combined with native soil microorganisms.

FIG. 5 is a graph setting forth the relative concentrations of variousgases present within a soil/gel microcosm.

FIGS. 6A and 6B set forth aspects of partitioning/diffusion rates anddistances within a thixotropic gel within a specific time frame.

FIGS. 7A and 7B set forth graphs showing viscosity parameter fittingvalues of various thixotropic gel solutions.

FIG. 8 and FIG. 9 illustrate calculations used to determine viscosityvalues for non-Newtonian (thixotropic) simulations.

FIG. 10 is a graph of a non-Newtonian saturation data comparinganalytical and actual values.

FIG. 11, FIGS. 12A and 12B, and FIGS. 13 through 16 set forth variousphysical property data on injected gel materials over time.

FIG. 17 and FIG. 18 set forth thixotropic gel and non-thixotropic oildistributions following injection.

FIG. 19 is a drawing showing the diffusion gradients and migrationpatterns of various constituents and contaminants within the vadose zonerelative to the thixotropic gel.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the embodiments of theinvention, one or more examples of which are set forth below. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncover such modifications and variations as come within the scope of theappended claims and their equivalents. Other objects, features, andaspects of the present invention are disclosed in the following detaileddescription. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstructions.

In describing the various Figs. herein, the same reference numbers areused throughout to describe the same material, apparatus, or processpathway. To avoid redundancy, detailed descriptions of much of theapparatus once described in relation to a Fig. is not repeated in thedescriptions of subsequent Figs., although such apparatus or process islabeled with the same reference numbers.

Vadose zone contamination is a wide spread problem, the contaminationresulting from numerous industrial practices. While a vadose zonecontamination source is typically small in area compared to acorresponding ground water contamination plume, the vadose zonecontamination is at a higher concentration. Further, withoutremediation, the vadose zone contamination provides a contaminationsource for surrounding ground water which creates a long-term groundwater contamination problem. The vadose zone is also characterized byreduced microbial activity since moisture content and nutrientavailability is substantially less in the vadose zone.

Application of a gel within the vadose zone provides an improvedenvironment for microbial colonization since there is a higher watercontent. If desired, various nutritional supplements may be added to agel mix and will support a higher microbial population in the vadosezone than would otherwise occur. Heretofore, microbial bioremediationhas been mainly directed to ground water contamination or used in aboveground reactors in a pump-and-treat type system. The highly permeablenature of the vadose zone along with the low water content and aerobicconditions have limited the use of bioremediation as a primary techniquefor vadose zone treatment strategies for certain contaminants.

In accordance with this invention, it has been found that an oil/watermixture may be formed into a thixotropic injectable solution using fumedsilica (silicon dioxide, pyrogenic silica) as an amendment. Underagitation, the resulting suspension has very fluid and liquid propertieswhich lend to conventional below ground injection techniques forplacement within the vadose zone. Once injected in the vadose zone, theoil/water combination will form a thick gel which thereafter isresistant to downward migration through the permeable vadose zone.

By controlling the relative amounts of oil to water, the relative amountof fumed silica, and the pH of the mixture, keeping into accountpotential pH changes associated with materials (minerals) present withinthe vadose zone, one having ordinary skill in the art may provide for arange of thixotropic oil/water materials which will form gels of varyingviscosities and geochemical properties. Useful oils include soy beanoil, safflower oil, flaxseed oil, sunflower oil, corn oil, olive oil,peanut oil, cottonseed oil, mineral oil, animal oils, and combinationsthereof.

As set forth in FIG. 1, data from a sample gel formulation of 50 percentwater, 50 percent soy bean oil, and 1.4 percent by weight fumed silica(Cab-O-Sli™) is used to construct gel containing soil samples includingnative microorganisms using soil samples collected from a depth ofbetween about 18 to about 21 feet from a non-contaminated location onthe property under jurisdiction of the Savannah River NationalLaboratory. Each sample microcosm consisted of 2 cm³ soil and 1 mil ofthe appropriate liquid (gel, water, soybean oil). The microcosms wereprovided with a 20 mil head space in a vial having septum seals to aidin gas chromatography analysis.

As seen in reference to FIG. 1, the gel formulations help establishanaerobic conditions within a vadose zone in comparison to samples usingonly pure water or neat soy bean oil with natural soil water content.

Set forth in FIG. 2, the conditions described above in FIG. 1 are usedand evaluated for the evolution of venting gases. Venting gas is ameasurement of microbial efficiency in converting the soy bean oilcarbon source. As seen in FIG. 2, the respective gels show significantimprovement in metabolism efficiency by microorganisms in comparison toneat soy bean oil or pure water. The decrease and then increase in theventing gas after day 140 is indicative of the natural progression ofcolonization by anaerobic microbes.

In addition to oil and gel, microcosm studies were conducting usingvariations of the types of gels as set forth in Table 1.

TABLE 1 Sample Designation Additive Water (0.5 ml) DI water only Oil(0.5 ml) Soybean oil only Gel (1 ml) 100 ml DI Water 100 ml Soybean Oil4 g Cab-O-Sil (2%) Yeast (1 ml) 100 ml DI Water (Nutrient 1) 100 mlSoybean Oil 4.07 g Cab-O-Sil (2%) 2.07 g Yeast Extract Corn (1 ml) 100ml DI Water (Nutrient 2) 100 ml Soybean Oil 4.05 g Cab-O-Sil (2%) 2.72 gCorn Steep Powder Combo (1 ml) 100 ml DI Water (Nutrient 3) 100 mlSoybean Oil 3.01 g Cab-O-Sil (2%) 1.42 g Corn Steep Powder 1.01 g YeastExtract

Evaluative microcosms were built using random soil core samples from 20to 25 feet of core samples from tetrachlorethylene (PCE) contaminatedsoil from a site under the jurisdiction of the Savannah River NationalLaboratory. Each microcosm had approximately 2 cm³ soil and theindicated volume of liquid identified in Table 1. Each microcosm had a20 mil head space provided by a vial having a septum seal to facilitategas chromatography analysis. It is noted that yeast extract and cornsteep liquor powder were introduced as a source of nitrogen, phosphorus,and other micronutrients.

As set forth in FIG. 3, the data sets forth that the gel formation withnutrients provides a more rapid use of oxygen in comparison to gelwithout nutrients. In the nutrient containing samples, oxygen iscompletely depleted within two weeks. Since one approach of using athixotropic gel within Vadose zones is to create an anaerobicenvironment, the inclusion of certain nutrients within the gel enablesthe anaerobic conditions to be established more quickly. In addition, itis possible to create an injectable gel which is anaerobic at the timeof injection. For instance, sparging the thixotropic solution withnitrogen gas so that the solution is saturated with nitrogen will createa substantially anaerobic solution which is devoid of free oxygen.

As seen in reference to FIG. 4, in comparison with FIG. 2, it is notedthat pentane is generated at a slower rate since available oxygen isquickly depleted in samples having the supplemental nutrients. Thegeneration of pentane is also indicative of the natural progression ofcolonization by anaerobic microbes. The slower metabolism of the soybeanoil under anaerobic conditions increases the longevity of the gel as atreatment and barrier system.

FIG. 5 sets forth the gaseous breakdown products in the nutrient gelhoused within a soil microcosm. The components and relativeconcentrations of gaseous breakdown products is indicative of thenatural progression from aerobic to anaerobic activity in the microcosm.It should be noted that separate measurements for oxygen are notreported, oxygen levels being below the detection limits and likelynonexistent.

Partitioning and diffusion tests were also conducted to determine theeffect of diffusion and partitioning on the overall process. Smallcolumns were created that had a constant vapor concentration at the topof PCE (tetrachloroethylene) having a PCE concentration at vaporpressure. The experimental apparatus used support members to hold 3plastic 5 mil syringes upright, a 10 mil volumetric flask, and a 2 quartglass jar. An oil gel of 50% water, 50% soy bean oil, and 1.2%/wt ofCab-O-Sil was used to construct the gel. The tops of the syringes wereremoved, the plungers pulled all the way out and filled with the oilgel. A 10 ml volumetric flask was then filled with PCE to the 10 ml markand the flask was placed in the jar and then the syringes were placed inthe jar with the flask. The jar was subsequently sealed and the 3systems were sampled and analyzed for PCE concentrations at 7 dayintervals as set forth in FIGS. 6A and 6B.

These data show a significant amount of PCE mass partitioning anddiffusing into the gel formulation in short time period where activesequestration and degradation can occur. Other non-polar contaminantswill behave similarly at different rates depending on their physical andchemical characteristics. These data show that once the gel has formedin the vadose zone, the oil fraction of the gel acts to absorb andsequester surrounding non-polar contaminants such as chlorinatedsolvents. The oil fraction of the gel continues to function as a sinkfor contaminants as the contaminants migrate through the mass of the gelby partitioning and diffusion. The ongoing sink maintains a beneficialdiffusion gradient in the vadose zone such that contaminants will movefrom a higher concentration within the vadose zone to the oil componentwithin the thixotropic gel.

Gel saturations useful for remediation are typically above 50 percentsaturation. The desired at rest viscosity may be adjusted based uponsoil permeability/porosity so as to meet the intended gel saturationlevels. Higher permeability/porosity soils will require higher at restviscosities to meet the intended saturation. Likewise, lowerpermeability/porosity soils would necessitate lower at rest viscosities.The at rest viscosity may be controlled by the amount of fumed silicawhich is incorporated into the gel.

By way of example, numerical modeling indicates that a medium sand witha moderate permeability requires an rest viscosity in the range of100,000 to 500,000 cp for saturations of 55 to 85 percent. This could bemet by a gel having an at rest viscosity of 50 percent non-toxic oilsuch as soybean oil and 50 percent water by volume in which 2 percentfumed silica such as Cab-O-Sil M5 (Cabot Corporation) by weight. Thefumed silica is mixed under high shear rates with either the oil orwater for an interval ranging from about 2 to about 8 minutes so as tocreate a homogeneous mixture. The remaining component of either oil orwater is then added and mixed under high shear rates for about 2 toabout 8 minutes to form a stable gel. The mixed formulation is stableand can be stored for significant time periods prior to injection intothe vadose zone.

The vadose zone soils tested have a pH value which ranges from about 4to about 5. Such pH ranges are too acidic and therefore detrimental tomicrobial processes. Therefore, low pH soils will require adjustment foroptimal microbial activity. In addition, anoxic conditions and increasesof carbon dioxide from microbial respiration can further decrease the pHin the soil. To promote microbial growth, a pH range should be nearneutral though ranges between about 5 to about 9 following emplacementof the gel within the soil system can be used. To raise the pH, a buffersuch as sodium bicarbonate, phosphates, sodium hydroxide or acombination thereof can be used in the water phase of the gel. Forspecific soils tested, 3×10⁻⁴ mol/l of trisodium phosphate will raisethe pH from 4 to a value greater than 6. One having ordinary skill inthe art will recognize that the buffer concentration may be tailored toaddress specific vadose zone conditions.

Microbial cell mass growth requires a carbon source such as soybean oilalong with oxygen, phosphorus, nitrogen, and various micronutrients. Thegel composition and additives may be adjusted and controlled todetermine the amount of resulting biomass which is formed. The remainingcarbon source is used by the microbes for respiration to degrade thecontaminants. The specific nutrient amendments of the gel may beadjusted to obtain a desired biomass to meet the bioremediation goalsfor the specific contaminants. Oversupply of nutrients can causeovergrowth of the microbes and clogging of the soil formation that canlimit mass transfer of certain contaminants and increase the degradationrate of the oil. For the soil conditions and gel recipe described above,nitrogen and phosphorous ratios were added to convert 1 to 2% of thecarbon in the soybean oil to biomass. The following stoichiometricequation is used to determine the appropriate ratios of nutrients toachieve the desired biomass per mass of soil to tailor the treatment forthe subsurface conditions and suite of contaminants:soybean oil+phosphorus+oxygen+nitrogen=cell mass3C₁₈H₃₂O₂+P+8O₂+6N₂≈C₆₀H₈₇O₂₃N₁₂PThe remaining carbon is used for microbial respiration and will increasethe longevity of the gel in the subsurface for treatment. One havingordinary skill in the art will recognize that the nutrient ratios may betailored to address specific remediation goals.

Macronutrients can be supplied in the form of organic powders orliquids, inorganic salts and/or organic or inorganic fertilizers. Whiledesired inorganic nutrients can be supplied to the injectable solution,the non-limiting examples set forth herein utilize the inclusion oforganic materials which, in response to metabolic activity bymicroorganisms, will provide for a gradual release of certain nutrientssuch as nitrogen and phosphorus.

In addition, at the time of application to the vadose zone, othersupplements including microorganisms, and growth supplements for themicroorganisms may be added, the microorganisms selected for theparticular contaminants to be treated. For instance, if an anaerobicthixotropic gel is being utilized, appropriate anaerobic microorganismscan be entrained into the thixotropic solution prior to injection in thevadose zone. Once the gel has formed in the vadose zone, the oilfraction of the gel acts to absorb and sequester surrounding non-polarcontaminants such as chlorinated solvents. The oil fraction of the gelcontinues to function as a sink for contaminants as ongoing microbialdegradation of VOCs, chlorinated solvents, and other contaminants occur.The ongoing degradation maintains a beneficial diffusion gradient in thevadose zone such that contaminants will move from a higher concentrationwithin the vadose zone to the oil component within the thixotropic gelas illustrated in FIG. 19.

The stabilized injected gel also maintains higher water content withinthe gel which supports enhanced levels of microbial activity. Thecombination of improved microbial conditions along with the increasedconcentration of contaminants allows for an effective combination ofbioremediation to occur.

Additional benefits of the applied vadose gel are that the gel acts as abarrier to intercept and incorporate downward migration of rain waterthrough the vadose zone. By preventing/limiting the downward migrationthrough the vadose zone, additional contamination of the ground water isminimized. As a result, the contaminants are sequestered in a smallerarea and in a substrate that lends itself to bioremediation. Further,the gel has a much greater permanence, i.e., non-migratory, thannon-thixotropic materials such as neat vegetable oils that have beenused in the past. This provides a greater resistance to the thixotropicgel and allows for a longer duration of beneficial reaction between thegel, colonized microorganisms, and contaminants which migrate into orare associated with the injected gel.

The thixotropic material also lends itself as a material and a processuseful for the remediation of heavy metals and metal contaminants. It iswell known in the art that numerous in situ technologies can be utilizedto remediate heavy metal contamination. Such techniques includeestablishing favorable pH environments in the subsurface soils, theaddition of chelating agents to precipitate heavy metals, and theintroduction and/or facilitation of microbial populations whosemetabolic activities will transform heavy metals into a less migratorychemical species or into a chemical form that is less harmful. Thethixotropic gel provides a useful matrix in which a favorable vadosezone environment for the sequestration and/or transformation of heavymetals may occur.

For instance, the gel matrix can be sufficiently buffered so as toprovide favorable pH ranges needed to bring about chelation orprecipitation of heavy metals. Further, useful bacteria that haveestablished themselves as effective in bioremediation of heavy metalscan be entrained into the thixotropic solution prior to injection. Thecombination of the desired microorganisms, beneficial pH, and any othersupplemental nutritional or reactive agents useful in heavy metalremediation can be included into the thixotropic gel material.

By way of example, the thixotropic material may include oxygen releasingmetal peroxides as taught in U.S. Pat. No. 7,160,471 which is commonlyassigned and which is incorporated herein by reference. In addition,various bacterial strains as disclosed in U.S. Pat. No. 6,923,914assigned to Global Biosciences, Inc., can be incorporated into thethixotropic solution to bring about remediation of heavy metals in situincluding aerobic, anaerobic, or dual aerobic/anaerobic conditions. U.S.Pat. No. 6,923,914 is incorporated herein by reference.

Injection simulations were conducted to investigate potential subsurfacedistribution patterns and flow behavior of VOS (Vadose OilSubstrate—thixotropic gel) under field conditions. A numerical modelcapable of simulating multiphase flow of a power law, non-Newtonianfluid (thixotropic) was developed by modifying T2VOC source code. T2VOCis a fully implicate, integral finite difference simulator formultiphase flow and heat transfer in porous media (Falta et al., 1995).As a general rule, the injected gel is distributed within the vadosezone along the path of least resistance. Accordingly, fractures that mayoccur within the vadose zone will tend to be filled by the injectedthixotropic gel material. Likewise, where a more porous soil, such asloose sand, is present, a greater penetration of the thixotropic gelwill occur in the more porous substrate as opposed to surrounding,denser soil layers. Irrespective of initial soil porosity, the injectedgel demonstrates a great deal of permanency with respect to downward orlateral flow as seen in neat vegetable oil substrates which tend tomigrate in response to gravity or along less permeable geologicformations within the subsurface.

Viscosity (μ) is a measure of the resistance of a fluid to deform (shearrate (γ)) under a stress (shear stress (τ)). Viscosity remains constantunder different shear stress conditions for most fluids. These fluidsare termed Newtonian, and the shear viscosity-shear stress relationshipcan be described as

$\mu = \frac{\pi}{\lambda}$

VOS gels are power-law non-Newtonian fluids. Non-Newtonian fluids arethose for which viscosity changes as a function of applied shear rate.For a power-law non-Newtonian fluid the relationship between shearstress and shear rate is described asτ=Hγ ^(n)and according to Ikoku and Ramey, 1980μ=Hγ^((n-1))  (1)where H is the consistency index and n is the flow index. H and n arecurve fitting parameters that are obtained by fitting viscosity versesshear rate data.

This evaluation was composed of 3 parts. One part involved preparing gelmixtures and testing viscosity properties. Another involved developingthe numerical code. Lastly the code was implemented to investigatesubsurface behavior of VOS under field conditions.

Laboratory Tests

The thixotropic gel was prepared by mixing 1:1 ratios of water andsoybean oil along with various amounts of Cab-O-Sil™ fumed silica. Threemixtures were prepared, each containing 500 ml of water and 500 ml ofvegetable oil. The different mixtures contained 6, 12, and 18 g ofCab-O-Sil, labeled 0.6%, 1.25%, and 1.86% respectively based on the massratio of Cab-o-Sil contained (vegetable oil density=0.9 g/ml). The 0.6%mixture contained too little Cab-o-Sil to retain a homogeneousconsistency after preparation and was not tested.

A Brookfield Gardner DVE Viscometer (model LV) was used to measure fluidviscosity at various applied shear stresses using spindles LV-2C andLV-3C (Table 2). The data was then fit using Equation 1 to obtain valuesfor H and n (FIGS. 7A & 7B).

TABLE 2 Shear Rate (s⁻¹) Viscosity (Pa*s) 1.25% 0.11 5.03 0.13 4.83 0.213.88 0.32 1.95 0.42 1.38 0.53 1.20 0.64 0.88 0.66 1.07 0.85 0.68 0.880.88 1.06 0.54 1.10 0.73 1.27 0.46 1.32 0.62 2.12 0.33 2.20 0.41 2.540.29 2.64 0.34 4.24 0.17 4.40 0.24 6.36 0.13 6.60 0.16 10.60 0.09 11.000.11 12.72 0.08 13.20 0.10 22.00 0.07 1.86% 1.07 30.96 1.02 20.37 0.824.27 0.41 2.09 0.29 1.03 0.25 0.65 0.19 0.41 0.23 0.22Model Development

Modifications were made to T2VOC source code to enable simulation ofpower-law non-Newtonian fluid flow in porous medium. The methodologyimplemented is based on that presented in Wu and Pruess, 1997.

The viscosity of a non-Newtonian fluid is a function of shear rate.Shear rate cannot be directly calculated within the framework ofexisting numerical flow simulators, however, Gogarty, 1967 has shownthat shear rate depends solely on fluid pore velocity in porousmaterials. The pore velocity is a function of local potential gradient,permeability, and, in multiphase systems, fluid saturation. Themultiphase extension of the non-Newtonian power-law function is

$\begin{matrix}{\mu = {\mu_{eff}( {\frac{{kk}_{r}}{\mu_{eff}}( {{\nabla\;\Phi}} )} )}^{\frac{n - 1}{n}}} & (2) \\{{and},{\mu_{eff} = {\frac{H}{12}{( {9 + \frac{3}{n}} )^{n}\lbrack {150\;{kk}_{r}{\phi( {S - S_{ir}} )}} \rbrack}^{\frac{1 - n}{2}}}}} & (3)\end{matrix}$where k is formation permeability, k_(r) is relative permeability, ∇Φ ispotential gradient, φ is porosity, S is non-Newtonian saturation, andS_(ir) is irreducible non-Newtonian fluid saturation.

As potential gradient approaches zero the viscosity from Equation 2 goesto infinity. Therefore, to maintain numerical stability viscosities forgiven permeability and saturation conditions are linearly extrapolatedfrom some small gradient back to zero gradient. This is done using

$\begin{matrix}{{\mu = {\mu_{1} + {\frac{\mu_{1} - \mu_{2}}{\delta_{1} - \delta_{2}}( {{{\nabla\;\Phi}} - \delta_{1}} )}}}{{and},{\mu_{j} = {{\mu_{eff}( {\frac{k}{\mu_{eff}}\delta_{j}} )}^{\frac{n - 1}{n}}( {{j = 1},2} )}}}} & (4)\end{matrix}$where δ₁ is slightly greater than δ₂. For this work δ₁ was assigned avalue of 10 Pa/m and δ₂ was assigned a value of 9.999999 Pa/m. Thelinear extrapolation method was applied when ∇Φ was less than or equalto 10 Pa/m. Due to the numeric storage limitations for positive doubleprecision values on a 64-bit processor (2.2E±308), lower limits had tobe placed on S and k_(r) values so that Equation 3 would not produce anundefined result (zero raised to a negative power). Therefore, S andk_(r) values that were less than 1E-15 were assigned values of 1E-15.This prevents the problem described above but has a negligible effect onthe viscosity calculation because under these conditions the potentialgradient would be small and Equation 4 is implemented.

T2VOC calculates the viscosity for Newtonian fluids as a function oftemperature for each individual volume element. To simulatenon-Newtonian flow, however, viscosity had to be calculated for eachvolume element interface because the potential gradient must be known toimplement Equation 2. A method had to be created for assigning eachvolume element a viscosity value in order to present spatial viscositydistribution results from simulations. In the cases presented eachvolume element was assigned the lowest viscosity value calculated forany of the volume element interfaces (FIG. 8). This basically equates toan upstream (with respect to gradient vector direction) weighting ofviscosity values.

T2VOC calculates gradients perpendicular to grid cell interfaces toobtain phase fluxes across those interfaces. These gradients, however,represent a component of the total gradient across a grid cell.According to Equation 2 viscosity of a power law fluid is a function ofthe total gradient at any given point. Therefore the total gradient hadto be estimated at each grid cell interface to estimate viscosity(gradient components are still used to calculate fluxes). Thecomplementary gradient component was obtained by dividing the pressuredifferential of the grid cell connections to either side by the distancebetween the points (FIG. 9). The total gradient is then calculated usingPythagoreans Theorem (FIG. 9).

The model validation process involved a simulation of Newtonian fluiddisplacement by a non-Newtonian fluid. The results of the simulationwere compared to the results of an analytical solution from an extensionof the Buckley-Leverett method (Wu et al, 1991). The numericalsimulation is a reproduction of the verification method used in Wu andPruess, 1997. The grid is one dimensional with 720 grid blocks that are1 m in the y and z-directions and 0.0125 m in the x-direction. Theinjection, formation, and fluid properties used are given in Table 3.Capillary effects were assumed to be negligible.

Comparison of the two resulting data sets shows that the numerical modelclosely predicts the results of the analytical model (FIG. 10). Thereis, however, slight disagreement near the “wetting” front of thenon-Newtonian fluid. This is due to numerical dispersion and is to beexpected.

TABLE 3 Porosity = 0.2 Permeability = 1 Darcy Injection rate = 8.233E-5m³s⁻¹ Injection time = 10 h Newtonian fluid viscosity = 5 cP IrreducibleNewtonian saturation = 0.2 Irreducible non-Newtonian saturation = 0 Flowindex = 0.5 Consistency index = 0.01 Pa*s^(n) Relative permeability ofnon-Newtonian phase = 1.17(S)² Relative permeability of Newtonian phase= 0.75(1-1.25S )² note (S is non-Newtonian fluid saturation)Model Properties

The same 2-D radial grid was used for all simulations. The grid iscomposed of 76 columns comprising a 121 m radius and 79 rows comprisinga depth of 42 m with 1 m of atmosphere located at the top. The 5 ft (1.5m) well screen is represented by the first grid blocks in 15-0.1 m rowslocated from 14.5 to 16 m (13.5 to 15 m bgs). The 17 rows above andbelow the well screen location are assigned thicknesses of 0.1 m andthen become thicker (up to 1 m) in either direction. The columns up to a5 meter radius are 0.1 m wide and then widen (up to 20 m) to a radius of121 m.

Initial pressure and saturation conditions were estimated for each gridblock assuming that the water table is located at 45 m bgs and thatatmospheric pressure is equal to 103100 Pa. The model was then run for asimulation time of 30,000 years to ensure that a very nearly steadystate water saturation distribution was reached. The steady stateinitial conditions were then incorporated into the model and the top row(atmosphere), bottom row, and largest radius column grid blocks were setto a constant condition state for all simulations.

Five different representations of formation material were used duringmodeling for this investigation. A range of formation materials fromsandy to clayey were represented (permeabilities=1×10⁻¹¹, 1×10⁻¹²,1×10⁻¹³, 1×10⁻¹⁴, and 1×10⁻¹⁵ m²). In each case the capillary pressurefunction parameters were adjusted to simulate actual properties for eachmaterial type. When making reference to a particular formation materialthe nomenclature used is 1E-11 m², 1E-12 m², etc.

Modeling Results and Discussion

Numerical models were designed to characterize the behavior of VOS inthe subsurface during injection and post-injection drainage periodsaround a conventional well with a 5-ft screen. When appropriate theresults from VOS simulations are compared to a baseline case. Thebaseline case involves injection of 100% vegetable oil, which wasexperimentally determined to be a Newtonian fluid with a viscosity of 46cP. The viscosity parameters used for the gel injection simulationscorrelate to those obtained from fitting the 1.25% mixture (H=0.74,n=0.11) (FIGS. 7A & 7B). It was determined that the 1.86% mixture is tooviscous to facilitate practical field application; therefore,simulations for this mixture were not performed.

Constant Pressure Injection Simulations

Simulations were conducted that predicted flow rates into a well, theviscosity distribution, and the saturation distribution of VOS andvegetable oil during constant pressure injection (FIGS. 11, 12A, 12B,13, 14). 10 hour long injections were simulated at 15 and 30 psiassuming a sandy type of formation material (1E-12 m²). Thesesimulations were constructed assuming that a sand pack was not present.The pressure indicated represents the simulated pressure at the top ofthe well screen, therefore, the fluid density and well screen depth mustbe considered when estimating the actual pressure applied at groundsurface.

In both the 15 and 30 psi cases the VOS and vegetable oil flow rateswere high at early times and then decreased as injection continued (FIG.11). The rate at which the VOS flow rates decreased was consistentlygreater than the rate at which the vegetable oil flow rates decreased.VOS flow rates were actually greater than the vegetable oil flow ratesat very early times, however, within a short period of time (30 min to 1hr) the VOS flow rates decreased to 10 to 20% of the vegetable oil flowrate under the same injection conditions (FIG. 11).

Decreasing flow rate with time is to be expected when injecting a fluidinto the unsaturated zone. When injecting a fluid under these conditionsthe volume of fluid saturated formation around the well screen increaseswith time. As this happens, the fluid mass that must be displaced bynewly injected fluid increases. This causes the flow rate to decreaseunder constant pressure conditions. This trend is more pronounced duringthixotropic gel injection simulations due to the method used tocalculate viscosity (Equation 2). According to this method viscosity hasan inverse-exponential relationship with potential gradient (exponent isnegative). When injection is initialized the potential gradient at thewell screen-formation interface is very large, therefore, the fluidviscosity is very low and the flow rate is large. As injection timeincreases the radius of fluid saturated formation around the well screenincreases. The injection pressure is constant, and the fluid pressure atthe wetting front of the fluid saturated formation volume remainsconstant (near atmospheric pressure). This means that the total pressuredrop over the saturated radius remains constant, therefore, as radiusincreases the overall average gradient decreases. Decreases in gradientcause increases in fluid viscosity (Equation 2) and, therefore,decreases in flow rate.

FIGS. 12A, 12B, and 13 show plots of property values with radius at alocation near the center of the well screen after 10 hours of injection.The overall fluid viscosity in the system increases with time due to thescenario described above, however, viscosity also increases with radiusaway from the well screen (FIGS. 12A & 12B). Overall, the viscosityvalues for VOS during the 30 psi simulation are less than those from the15 psi simulation due to an increase in the overall potential gradientin the system. In both cases the viscosity increases with radius awayfrom the well screen with an abrupt jump near the gel-wetting front(FIG. 12 a). The jump is due to the fact that viscosity has aninverse-exponential relationship with relative permeability (Equation2). Near the wetting front the fluid saturation decreases, causing adecrease in relative permeability and thus an increase in fluidviscosity. If the abrupt jump in viscosity near the wetting front isignored, plots show that viscosity increases with radius (FIG. 12 b)over the region where fluid saturation remains fairly constant (FIG.13). This can be explained by observing how fluid pressure changes withradius (FIG. 12 b). Fluid pressure decreases as radius increases.

For purposes of calculating viscosity, however, it is not the value offluid pressure that is important. Rather, it is the slope of the fluidpressure with radius line at any given point that dictates viscosityunder constant saturation (and therefore constant relative permeability)conditions. The slope (potential gradient) slightly decreases withradius. Small differences in slope cause comparatively large changes influid viscosity (FIG. 12 b) due to the exponential relationship betweenpotential gradient and viscosity in power-law fluids (Equation 2). Theviscosity for Newtonian fluids such as vegetable oil is not affected byshear stress, and therefore remains constant (FIGS. 12A & 12B).

As VOS or vegetable oil is injected into a formation it fills porespaces by displacing water and air. The assumed wetting phase hierarchyin T2VOC simulations is Water>NAPL>Air. Therefore air is completelydisplaced by a NAPL (VOS, vegetable oil) phase, whereas water is not.The saturation that remains after a NAPL wetting front moves through aparticular volume of formation material is dependent upon the minimumresidual water content and capillary pressure properties of theformation. In this case a permeable formation material is believed tooccur where the minimum residual water content is zero and the resultingcapillary pressure differences are small. Therefore, the NAPLsaturations behind the wetting front are large (0.97) with capillarityaccounting for a small water saturation (0.03) (FIG. 13).

The maximum radius reached by an injected fluid is dependent on theaverage flow rate over the injection period, and to a lesser extent onthe geometry of the fluid bearing volume of formation. Average flow ratedecreases as injection pressure decreases; therefore, in both cases themaximum radii reached are larger for the 30 psi injections than for the15 psi injections (FIG. 13). The fluid viscosity during injection isgreater on average for VOS than for vegetable oil. This accounts for theflow rate differences (FIG. 11) that allow vegetable oil to reach largerradii under the same injection conditions (FIG. 13). Ten hours ofinjection at 15 psi produces radii of 0.35 and 0.65 m for VOS andvegetable oil respectively, whereas ten hours of injection at 30 psiproduces radii of 0.65 and 0.85 m respectively (FIG. 13).

Effects of Formation Permeability

The radial distance reached by VOS under various injection conditions isof particular interest. A set of homogeneous formation simulations werecreated using the four different sets of formation material propertiesto investigate the affect of formation material composition onsaturation with radial distance from the well. Simulations involvedinjecting VOS at 30 psi over a period of 10 hours.

In general the flow rate with time results based on formation materialtype were as expected, in that the overall flow rate decreased asformation permeability decreased (FIG. 14). In each case the flow rateis greater at early times and decreases as injection continues. The rateat which flow decreases during early times is slightly greater for thelower permeability cases (1E-13 and 1E-14 m²). The most likelyexplanation for this difference is the varying effect of capillarypressure function parameters, porosity, and residual water saturation,which are more extreme for these materials.

The overall average value of fluid viscosity increases as formationpermeability decreases when injecting a shear-thinning power law fluid(FIG. 15). This is due in part to the fact that viscosity has an inverseexponential relationship with permeability (Equation 2). The gradient atany given point in the NAPL bearing volume also decreases withpermeability because the entire volume up to the wetting front ismaintained at a higher pressure (abrupt drop in pressure at the wettingfront).

As stated previously, the maximum NAPL saturation that can be obtainedis a function of residual formation water saturation and capillarypressure properties. The effect of this relationship can be observed inFIG. 16. The residual water saturation and capillary pressure effectstypically increase as formation permeability decreases. Thereforemaximum VOS saturations obtained decrease with formation permeability.

Effects of Anisotropy

The subsurface can very rarely be classified as homogeneous andisotropic. Most often there is some degree of anisotropy, typically inthe form of alternating layers of formation materials with differingdegrees of permeability. To investigate the effects of anisotropy,models were created to simulate two different conditions involvinglayering with permeability contrast. The formation materials in bothcases were combinations of the 1E-12 and 1E-14 m² types. One simulationassumed that the 5 ft screened interval was completed exactly within a 5ft thick horizontal high permeability zone (1E-12) capped on top andbottom with a lower permeability formation (1E-14). The other variationinvolved having the well screen intersect two horizontal highpermeability zones, a 1 ft thick layer at the top of the well screen anda 2 ft thick layer at the bottom of the well screen.

It is believed that injecting into a “confined” high permeability layerwould possibly increase the maximum radius reached by the injected gel.This possibility was considered probable because under these conditionsflow in the vertical direction (upward or downward) away from the wellscreen is limited, therefore isolating the effective force from theconstant pressure source in a solely horizontal direction. The resultsof the anisotropic simulations, however, suggest that this may not bethe case. In both the 1 and 2 layer simulations the maximum radiusreached is approximately 0.65 m (FIG. 17). This is similar to themaximum radius reached during the 30 psi homogeneous simulation thatused the 1 e-12 m² formation material (FIG. 13).

The VOS saturation decreases vertically downward within each layer atany given radius (FIG. 17). This can be attributed to changes in how theequilibrium saturation profile develops within high permeabilityformation materials that are in contact with low permeability units.Water within the formation in a high permeability, homogeneous case willtend to drain downward and, assuming that the water table is asignificant distance away, the water saturation within the formationwill tend to approach the residual saturation (0 in the case of 1E-12).However, when a high permeability material overlies a lower permeabilitymaterial the draining water tends to “pool” on the interface between thecontrasting materials. This creates a steepened water saturation profileover the thickness of the high permeability unit. Higher watersaturations cause decreases in NAPL relative permeability and,therefore, decreases in flow rates and NAPL saturations near the wettingfront. These effects are more pronounced near the bottom of the layerswhere water saturations are greater (FIG. 17).

Drainage Simulations

Simulations were created to investigate fluid behavior differences inthe subsurface after injection is completed. This was accomplished bysimulating constant flow rate injection into a homogeneous formation forboth VOS and vegetable oil. The 1E-12 m² formation material was used inboth simulations.

The injection rate in each case was 10 L/m over a period of 10 hr for atotal of 6000 L (1585 gal). After this 10 hour period was completeinjection was ceased and a drainage period of 1 year was simulated(FIGS. 18A & 18B). NAPL partitioning into the aqueous and gaseous phasesis ignored for these simulations.

In both cases the NAPL saturations within the NAPL bearing zones arehigh (>0.95). Observation of the vegetable oil simulation results showsthat vegetable oil drains fairly readily. Within a month the lowestpoint of the wetting front has migrated nearly 2 meters verticallydownward and the maximum NAPL saturation has decreased to less than0.85. After a period of 1 year a 0.1 saturation portion of the wettingfront has migrated down 3 meters and the overall maximum NAPL saturationhas decreased to approximately 0.6 (FIG. 18B).

In the VOS case, however, there is negligible drainage even after 1year. The potential gradient provided by gravity in this case is notlarge enough to decrease viscosity to the point where any appreciableflow occurs. The viscosities within the NAPL bearing zone are all on theorder of 1×10⁷ to 1×10⁹ cP.

The preceding modeling exercise was limited to constant pressureinjection. As illustrated in the above discussion, increasing thepressure during constant pressure injection will increase the distancethe gel moves in the vadose zone. In addition, the gel can be injectedunder constant flow rate and under high pressure to create fracturesfilled with the gel in low permeability formations. One having ordinaryskill in the art will recognize that there are multiple ways forinjecting the thixotropic gel into the vadose zone.

Although preferred embodiments of the invention have been describedusing specific terms, devices, and methods, such description is forillustrative purposes only. The words used are words of descriptionrather than of limitation. It is to be understood that changes andvariations may be made by those of ordinary skill in the art withoutdeparting from the spirit or the scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged, both in whole, or in part. Therefore,the spirit and scope of the invention should not be limited to thedescription of the preferred versions contained therein.

1. A thixotropic gel for injecting into a subsurface soil comprising:about 40 percent to about 60 percent by composition volume of abiodegradable vegetable oil such as soybean oil or mineral oil andcombinations thereof; about 40 percent to about 60 percent bycomposition volume of water; which may be comprised of additional watersoluble carbon sources; and, fumed silica in an. amount of between about0.75 percent to about 2.0 percent by weight, wherein said thixotropicgel is substantially devoid of free oxygen.
 2. The thixotropic gelaccording to claim 1 wherein said gel further comprises nutritionalorganic supplements for supporting microbial growth selected from thegroup consisting of yeast extracts, vitamins, corn steep liquor, seaweedextracts, and organic and inorganic fertilizer species, and combinationsthereof.
 3. The thixotropic gel according to claim 1 wherein said gelfurther includes a supply of microorganisms adapted for in situbioremediation in a subsurface environment.
 4. The thixotropic gelaccording to claim 1 wherein said thixotropic gel contains a saturatedconcentration of nitrogen gas.
 5. The thixotropic gel according to claim1 wherein said thixotropic gel further comprising a pH-buffering agent.6. A thixotropic gel for injecting into a subsurface soil comprising:about 40 percent to about 60 percent by composition volume of abiodegradable vegetable oil such as soybean oil or mineral oil andcombinations thereof; about 40 percent to about 60 percent bycomposition volume of water; and, fumed silica in an amount of betweenabout 0.75 percent to about 2.0 percent by weight, wherein saidthixotropic gel contains a saturated concentration of nitrogen gas. 7.The thixotropic gel according to claim 6 wherein said gel furthercomprises nutritional organic supplements for supporting microbialgrowth selected from the group consisting of yeast extracts, vitamins,corn steep liquor, seaweed extracts, and organic and inorganicfertilizer species, and combinations thereof.
 8. The thixotropic gelaccording to claim 6 wherein said gel further includes a supply ofmicroorganisms adapted for in situ bioremediation in a subsurfaceenvironment.
 9. The thixotropic gel according to claim 6 wherein saidthixotropic gel further comprising a pH-buffering agent.
 10. Athixotropic gel for injecting into a subsurface soil comprising: about40 percent to about 60 percent by composition volume of a biodegradablevegetable oil such as soybean oil or mineral oil and combinationsthereof; about 40 percent to about 60 percent by composition volume ofwater; and, fumed silica in an amount of between about 0.75 percent toabout 2.0 percent by weight; wherein said gel further includes a supplyof microorganisms adapted for in situ bioremediation in a subsurfaceenvironment.
 11. The thixotropic gel according to claim 10 wherein saidgel further comprises nutritional organic supplements for supportingmicrobial growth selected from the group consisting of yeast extracts,vitamins, corn steep liquor, seaweed extracts, and organic and inorganicfertilizer species, and combinations thereof.
 12. The thixotropic gelaccording to claim 10 wherein said thixotropic gel further comprising apH-buffering agent.